U.S. Sudan envoy Scott Gration is meeting government and opposition officials in Khartoum, trying to salvage the country's troubled election process. Opposition candidates say they will boycott the country's upcoming presidential election.

Officials here say Gration is in Sudan on a rescue mission for what are supposed to be the African country's first multi-party elections since 1986.

Gration left Washington earlier this week amid signs of an impending opposition boycott of the presidential voting due to begin April 11.

Since then, the SPLM, the ruling party of the semi-autonomous southern Sudan region said it was dropping out of the race because of alleged vote-rigging by the National Congress Party of President Omar al-Bashir.

Three other opposition parties said Thursday they have also withdrawn. This month's election is a critical part of the 2005 Comprehensive Peace Accord, the CPA, intended to resolve Sudan's long-running north-south conflict.

At a news briefing, State Department Spokesman P.J. Crowley suggested that Gration still believes the election can be salvaged, calling the political situation in Khartoum fluid and saying the announced decisions by the opposition groups are not necessarily final.

"Our goal is to produce as competitive an election as possible. We recognize that this is a difficult environment. These are complex and difficult issues," he said. "Sudan hasn't done this in a while. But we're aware of the issue, we're looking into it, we're working with the parties. We want to see full implementation of the CPA. But I just don't want to specifically say that what might be occurring at one moment is necessarily going to be the definitive, final answer," said Crowley.

Crowley said the presidential election is a major step toward the referendum to be held next year on the political future of southern Sudan, and that getting it right will be a big advance for both the north and south.

A senior State Department official who spoke to reporters said a boycott would not be in the long-term interests of Sudan's opposition parties, though he said he was not minimizing their complaints about the process.

The Brussels-based International Crisis Group Tuesday also accused the ruling National Congress Party of trying to rig the elections, saying the NCP has drafted election laws in its favor and that flawed census data had been used to set up electoral districts.

Human rights groups have said the ruling party is intimidating and attacking opponents.

In a joint statement on Tuesday, U.S. Secretary of State Hillary Clinton, British Foreign Secretary David Miliband and Norwegian Foreign Minister Jonas Gahr Stoere urged all Sudanese parties to work urgently to ensure that the April elections can proceed peacefully and credibly.

The three officials expressed deep concern about what they termed continuing administrative and logistical challenges to the voting, as well as restrictions on political freedoms.